White Paper: OLED Screens

Organic light-emitting diode (OLED) screens offer better picture quality and draw less power than traditional LCDs. But what are OLEDs?

Organic light-emitting diodes, or OLEDs, are often touted as the next big thing in display technology, offering brighter colors, true black, lower power consumption, and better off-axis viewing than traditional LCD screens. They’ve popped up in gadgets from high-concept to mundane: The infamous Optimus Maximus keyboard, for example, utilizes many tiny OLED screens in its programmable and customizable keycaps, and both Sony’s new X-series Walkman and Microsoft’s new Zune HD have OLED screens. OLED technology has made great strides in the past 10 years, and cheaper and better manufacturing processes mean they’ve started appearing in everything from media players to phones to high-definition televisions—even keyboards. But what are OLEDs?

What’s Inside

In the simplest terms, LEDs (light-emitting diodes) emit light by running an electrical current through a diode. Diodes create unidirectional electric flow, moving electrons from the negatively charged cathode to the positively charged anode, creating electron holes, or spaces where electrons could be. Electrons flowing in drop into these holes and emit light. An organic light-emitting diode uses the same principle, but between the cathode and anode are two layers of organic semiconductor compounds: the emissive layer, near the cathode, and the conductive layer, near the anode (organic compounds are chemical compounds that contain carbon). The cathode sends (negatively charged) electrons into the emissive layer, while the anode draws electrons from the conductive layer, leaving positively charged “electron holes.” This creates a negatively charged emissive layer and a positively charged conductive layer, which attract each other, drawing electron holes to the emissive layer. The positive-charged holes and negative-charged electrons recombine, lowering the energy levels of the electrons, emitting light as a by-product. Simple, right?

From a development standpoint, OLEDs have a lot of potential. Organic chemistry is a fairly well-understood science—reds, blues, and greens were developed in a much shorter time frame in OLEDs than in regular LEDs. And new molecules that can be used in the layers, which have longer lifetimes and produce brighter colors, are being discovered frequently.

In an OLED, an electrical current causes electrons (-) to move from the cathode to the emissive layer, creating a negative charge in the emissive layer. The positively charged anode attracts electrons from the conductive layer, creating a positive charge in the conductive layer, which recombine with holes (+) in the conductive layer attract electrons from the emissive layer, which recombine with the electron holes, lowering the energy level of the electrons and emitting light as a by-product.

It turns out that OLEDs are great for use in displays, because the organic molecules that comprise the emissive and conductive layers can be deposited in very thin, large sheets onto a variety of substrates—from glass to metal to fiber—so that millions of individual OLEDs can be crammed together, row by row and column by column, into a very small space. Each of these OLEDs becomes one pixel of the display. The organic compounds can be deposited using several methods, depending on the type of organic molecule used in the display.

There are two types of OLEDs currently in production and development, differentiated by the size of the molecules in their organic compounds. Small-molecule OLEDs are usually manufactured via organic vapor phase deposition (OVPD)—the organic molecules are evaporated and carried via inert gas, then deposited on a substrate through a series of very small nozzles held near the substrate’s surface. Large-molecule, or polymer OLEDs, can be created via a process similar to inkjet printing—the polymers are dissolved into a solution and “printed” onto the substrate.

Advantages & Disadvantages

The advantages of OLEDs over traditional LCDs are many. First, unlike liquid-crystal displays, OLED pixels actually emit light, so they don’t require backlighting. Traditional LCD screens often utilize traditional LEDs or CCFLs for backlighting, which—in addition to increasing the thickness of the display to accommodate a light source—prevents the display from rendering true black, as even “black” LCD pixels are backlit. Since OLED pixels produce light when on and don’t produce light (or draw power) when off, a darker, richer black can be created. Having light-emitting pixels also enables richer colors, a broader color gamut, higher contrast, and a greater viewing angle than an LCD screen. Because “off” pixels don’t draw power, and because there’s no need for a separate light source, OLED displays require less energy to run. And because the organic molecules can be printed onto a variety of substrates, flexible displays are possible.

OLEDs, however, are not without their disadvantages. The manufacturing process is still expensive, so large OLED displays are rare—most OLEDs are used in small-screen applications, such as media players and smartphones, though HD displays up to 40-inches have been demonstrated. And the materials used in OLEDs don’t necessarily last as long as regular LCD displays—another reason they’re more frequently found on phones and media players, rather than computer monitors and televisions. Monitors are typically turned on for much longer stretches of time. And finally, the organic materials in OLEDs are extremely susceptible to water damage, so displays must be well-sealed.

OLED to the Future

What’s next for OLED technology? The European Union, among others, is investigating the use of OLEDs as cheap solid-state lighting to replace incandescent bulbs. Their stated goal is to create a 100x100cm square of OLED material that creates 100 lumens per watt of power, has a working lifespan of at least 100,000 hours, and costs less than 100 euros per square meter to produce.

OLEDs have found their way into concept cars, lighting fixtures, PMPs, and laptop prototypes, with the latter expected to enter production by Q3 2010. As manufacturing processes become less expensive, OLED displays could start to replace LCDs, not just in media players and phones, but also in notebook computers, monitors, and televisions, on a much larger scale.